Congress meddled and forced Fannie and Freddie to lend more and more to people who could not afford houses -- and without charging anything for the extra risk. Fannie and Freddie failed because of Congress 100%

The Fed lowered rates to 1% under some dubious nonsense about deflation (and more accurately the need to bailout bad dot-com positions). They kept rates way too low for way too long.

Bank regulators were tripping over themselves to make sure banks lent to lower income individuals, hoping to let them cash in on the housing "bonanza" -- hoping that these people would not notice that their incomes were rising 1% while the cost of living was rising 7%.

The SEC joined in on the act by requiring buy side funds to subjugate credit analysis to the three credit agencies.

And the SEC joined with FASB to lower accounting standards to permit all sorts of off balance sheet financing (SIVs, CDOs, etc)

Whatever you think Wall Street did, they did it with the full knowledge, consent and sometimes encouragement of regulators